In a power semiconductor device, a surge voltage (overvoltage) greater than a withstand voltage is generated between a drain and a source, that is, in a drain-source path, of the power semiconductor device due to an inductive (L) load and noise, when it is turned off in the switching operation. This surge voltage sometimes causes breakdown of the power semiconductor device. A conventional power semiconductor device is generally made of silicon (Si) and a withstand voltage of such a power semiconductor device is limited by a limited characteristic or performance of Si material itself. As an alternative, a wide gap semiconductor device exemplified by gallium nitride (GaN) is considered. The GaN semiconductor device is promising as a power semiconductor device, which has a low on-resistance and a high withstand voltage.
Although the conventional Si semiconductor device (for example, MOSFET and IGBT) is configured to withstand surge voltages, the GaN semiconductor device itself is not configured so and hence has no surge voltage withstand property. It is therefore necessary to provide an external circuit to ensure the surge voltage withstand property, which is required to be used for vehicles.
JP 2000-077537A (patent document), which corresponds to U.S. Pat. No. 6,385,028, discloses a power semiconductor device, which improves surge voltage withstand property. FIG. 11 shows an exemplary power semiconductor device driving circuit. As shown in the figure, Zener diodes 102 are arranged for voltage clamping between a drain and a gate of a power semiconductor device 100 in addition to Zener diodes 101 arranged for gate protection between the gate and a source of the power semiconductor device 100. When a surge voltage is applied to the drain, the Zener diodes 102 break down and absorb the surge.
According to the driving circuit disclosed in the patent document, when the surge voltage in a drain-source path exceeds the Zener voltage at the time of turn-off of the power semiconductor device 100, its drain-gate path conducts and a gate voltage is raised. Thus the surge voltage is controlled to a sum of the Zener voltage and a threshold voltage of the power semiconductor device.
However, since a rated voltage of the Zener diode increases in a high power system, the driving circuit becomes large and costs high.
The GaN semiconductor device described above has a small capacitance and hence is more susceptible to parasitic capacitance than the conventional semiconductor device. For this reason, if a parasitic capacitance is added to the gate, it will lower high speed switching performance characteristic.